It has been known in the prior art to use pulsed laser systems to effect such processes as diverse as metal machining and biological tissue removal. Of chief concern in these systems is the amount of “collateral damage” to the surrounding regions of the workpiece, or, in the case of biological uses, surrounding tissues. In the case of the machining of metallic workpieces, for example, laser pulses greater than 100 microseconds in duration will machine the workpiece at the cost of creating a significant pool of molten liquid which is ejected from the beam impact site. Cleanly machined features cannot be obtained with this machining technique owing to the tendency of the molten material to spatter the workpiece and/or freeze and harden on the workpiece itself. This effect is due, of course, to the transfer of a significant amount of heat into the workpiece material at the target zone and at surrounding areas as well. In the case of biological procedures, this heat transfer effect typically causes unacceptable collateral damage to the surrounding tissues.
A general but partial solution to this problem resides in the use of shorter pulse durations. With shorter pulses the target is heated more quickly and thus reaches the evaporation point before significant liquid is permitted to form. Thus, in this arena, the shorter Q-switched temporal pulse may find advantage in certain applications. The pulse widths of conventional Q-switched, solid state lasers used in micro machining is approximately 50–200 nanoseconds. This pulse width has for many cases proven to provide a reasonable balance between laser cost, machining accuracy and collateral effects such as the size of the heat-affected zone (HAZ), it being generally understood that the cost of laser systems of significant power increases greatly with the shortness of the period of the output pulse.
However, even in the above mentioned pulse width range, the degree of heat transfer into the material is unacceptable for many applications. Recently developed lasers reported at OE/LASE SPIE vol. 2380 pp 138–143 (1995) which generate pulses in the 8–20 ns range abate this problem to a degree, however since the threshold for ablation in the nanosecond range decreases as the reciprocal of the square root of the laser temporal pulse width, it is apparent that as the pulsewidth is further reduced, the range of potential applications broadens considerably.
With advances in pulsed laser systems, lasers having pulse widths well into the femtosecond regime have become available. At these ultrashort pulse widths, collateral damage to surrounding regions becomes almost negligible, because of the lack of significant heat transfer into zones outside of the immediate target area. Essentially, the material at the target is substantially instantaneously vaporized while the fleeting duration of the impact of the laser energy substantially eliminates the possibility of heat transfer into surrounding areas. In general, it is known that the heat penetration depth L is proportional to the square root of the product of the heat diffusion coefficient (specific to the material) and the pulse width t. Consequently, as the pulse width becomes shorter, the heat penetration depth decreases proportionately. With femtosecond pulses, ablation thus takes place before significant heat can be transferred into the material, so that little or no heat effected zone (HAZ) is created. U.S. Pat. Nos. 5,656,186 and 5,720,894, incorporated herein by reference, discuss the above effects generally, and disclose laser systems operating well into the femtosecond regime in some instances.
However, as previously mentioned, the costs associated with femtosecond-regime micro-machining lasers are not insignificant; they presently cost five to fifteen times more than the present nanosecond-regime micro-machining sources. Thus, there is a need in the industrial and medical fields for a micro-machining or marking laser which reduces the collateral damage problems of the prior art, yet has a cost comparable to the present sources. This goal has been achieved through the present invention, which, through the use of a novel and highly efficient combination of Q-switching and Yb fiber laser techniques, provides a source operating in the short nanosecond or sub-nanosecond regime which is less expensive than the micro-machining sources now conventionally used, generating pulses as much as 4 orders of magnitude smaller than that in the known micromachining arts, and thus producing a greatly decreased heat affected zone which is practical for a wide variety of applications while avoiding the greatly increased cost of present femtosecond systems.
As mentioned above, Q-switching is currently a common technique for generating nanosecond optical pulses. It is known that the main parameter which determines the duration of a Q-switched laser pulse is the laser cavity round-trip time Tround-trip=2Lcavity/c, where c is the speed of light and Lcavity is the laser cavity length. Therefore, shorter laser cavity length is generally required for generating shorter Q-switched pulses. However, it is known that this shortening of the cavity length normally reduces the mode volume which makes if more difficult to achieve suitable pulse energies. Further amplification in a solid-state amplifier is usually not a practical solution due to the very low gain characteristic of solid-state amplifiers. Moreover, pushing the energies from a short pulse microchip laser sufficient for micromachining, reduces the microchip laser efficiencies to around 5%.
Here we demonstrate that by using a low energy microchip laser in conjunction with a highly efficient large core Yb fiber amplifier these problems can be overcome and subnanosecond optical pulses can be achieved at high pulse energies.
Known Nd: based lasers, in addition to being expensive, are less efficient compared to Yb-doped fiber amplifiers. For example, Nd:YAG lasers transform the diode pump power to optical output at approximately 50% efficiency. In contrast, Yb fiber amplifiers transform laser diode pump power to optical output with about 90% efficiency. This better efficiency leads to certain cost savings, especially when the comparison is based on cost per unit of output power.
The amplification of high peak-power and high-energy pulses in a diffraction-limited optical beam in single-mode (SM) optical fiber amplifiers is generally limited by the small fiber core size that needs to be employed to ensure SM operation of the fiber. To overcome the energy and peak power limitations, recently the use of multi-mode (MM) fiber amplifiers has been suggested (U.S. Pat. No. 5,818,630 to Fermann and Harter, herein incorporated by reference). In this work the loss of spatial beam quality in MM fiber amplifiers is prevented by excitation of the fundamental mode via the use of appropriate mode-matching bulk optics or fiber tapers as suggested in U.S. Ser. No. 09/199,728 to Fermann et al., herein incorporated by reference.
Particularly interesting are MM fiber amplifiers that are double-clad since they can be conveniently pumped with high-power diode lasers to produce high average powers. Moreover, the achievable small cladding/core ratio in double-clad MM fibers also allows the efficient operation of fiber lasers with small absorption cross sections, as suggested in the aforementioned U.S. Pat. No. 5,818,630 to Fermann and Harter.
Cladding-pumped fiber amplifiers and lasers have been known for many years. See U.S. Pat. No. 4,829,529 to J. D. Kafka, U.S. Pat. No. 4,815,079 to Snitzer et al., U.S. Pat. No. 5,854,865 to Goldberg, U.S. Pat. No. 5,864,644 to DiGiovanni et al., and U.S. Pat. No. 5,867,305 to Waarts et al. In the early work in this area (Kafka and Snitzer) only double-clad fiber amplifiers comprising a SM core were considered for cladding-pumping, resulting in obvious limitations for the amplification of high peak power pulses. Moreover, Snitzer et al. only considered double clad fibers with approximately rectangular-shaped or non-centrosymmetric cladding cross sections to optimize the absorption efficiency of such fibers. The use of relatively small cladding/core area ratios enabled by double-clad fibers with a large multi-mode core, however, allows for the efficient implementation of any arbitrary cladding cross section, i.e. circular, circular with an offset core, rectangular, hexagonal, gear-shaped, octagonal etc. The work by Kafka was equally restrictive in that it only considered double-clad fibers with a single-mode core pumped with coherent pump diode lasers. Again the use of relatively small cladding/core area ratios enabled by double-clad fibers with a large multi-mode core enables the efficient implementation of pump diode lasers with any degree of coherence.
The later work of Goldberg and DiGiovanni was not necessarily restricted to the use of double-clad fibers with SM fiber cores. However, none of the work by Goldberg and DiGiovanni (or Kafka, Snitzer or Waarts et al.) considered any technique for the effective use of multi-mode double-clad fibers as diffraction-limited or near diffraction-limited high-power amplifiers. No methods were described for exciting the fundamental mode in multi-mode amplifiers, no methods were described for minimizing mode-coupling in multi-mode amplifiers and no methods were described for controlling the excitation and the size of the fundamental mode by gain-guiding or by the implementation of an optimized distribution of the dopant ions inside the multi-mode fiber core.
Moreover, the specific pump injection technique suggested by DiGiovanni comprises built-in limitations for the efficiency of fundamental-mode excitation in multi-mode fiber amplifiers. DiGiovanni considers a fused taper bundle with a single-mode fiber pig-tail in the center of the bundle, which is then spliced to the double-clad amplifier fiber to simultaneously deliver both the pump light (via the outside fibers of the fused taper bundle) and the signal light (via the single-mode fiber pig-tail) to the amplifier fiber. Due to the limited packing ability of circular structures, air gaps remain in the fiber bundle before tapering. Once tapered, surface tension pulls all the fibers in the fiber bundle together, essentially eliminating the air gaps (as discussed by DiGiovanni et al.). As a result the outside cladding of the taper bundle becomes distorted (resulting in a non-circular shape with ridges where the fibers were touching and with valleys where there were air-gaps). Hence the central core region and the fundamental mode also become distorted which limits the excitation efficiency of the fundamental mode in a MM fiber when splicing the fiber bundle to the double-clad fiber. In fact any geometric differences in the cladding shape of the fiber bundle or the double-clad fiber will lead to a limited excitation efficiency of the fundamental mode in the MM fiber in the process of splicing.
For reducing size and cost of the system as well as for increasing efficiency of the amplification side-pumping (as described in aforementioned U.S. Pat. No. 5,818,630) rather than end-pumping might be advantageous. For the benefits of fiber reliability the use of fiber couplers is preferred. The use of fiber couplers for pump light injection into MM fibers is discussed in aforementioned U.S. Ser. No. 09/199,728.
Normally for many applications a single polarization is desirable, so the use of polarization preserving fiber is desirable. There are several means of making polarization preserving fiber. However, for multimode fiber, elliptical core fiber is the easiest to manufacture and to obtain at this time.
Another attractive feature would be ease of fiber coupling the laser to the application, by using the amplifier fiber as the fiber delivery system, or a multimode undoped fiber spliced to the end of the amplifier fiber. This is similar to the fiber delivery system described in U.S. Pat. No. 5,867,304 and its progeny, herein incorporated by reference, where a multimode fiber is used for delivery of a single mode beam. The purpose is to lower the intensity in the fiber by using the larger effective mode-field diameter. This allows higher peak powers; >1 KW pulses can be transmitted without the onset of nonlinear processes. In U.S. Pat. No. 5,867,304, this fiber is used with ultrashort pulses where the fiber dispersion distorts the pulses. However, with nanosecond pulses, dispersion has a negligible effect on the pulse width so dispersion compensation is not necessary.